How to Add and Subtract
Positive and Negative Numbers

Numbers Can be Positive or Negative:

Negative Numbers (-)	Positive Numbers (+)
(This is the Number Line, read about Using The Number Line)

"-" is the negative sign.	"+" is the positive

 

No Sign Means Positive

If a number has no sign it usually means that it is a positive number.

Adding Positive Numbers

Subtracting Positive Numbers

But what happens if we have negative numbers ... ?

For example, what is 6 - (-3) ?

Subtracting a Negative is the same as Adding

Ally can be naughty or nice. So Ally's parents have said "If you are nice we will add 3 points (+3). If you are naughty, we take away 3 points (-3). When you reach 30 Points you get a toy." Ally starts the day with 9 Points:

Ally's Mum discovers spilt milk: 9-3 = 6   Then Dad confesses he spilt the milk and writes "undo". Mum calculates: 6-(-3) = 6+3 = 9

So if you subtract a negative, you gain points
(ie the same as adding points).

What About Positive and Negative Together ... ?

You can also have questions like:

What is 6 + (-3) ?
or what is 6 - (+3) ?

They are really the same! Both are subtraction:

Subtracting a Positive or Adding a Negative is Subtraction

A few days later. Ally has 12 points.   Mum adds 3 points because Ally's room is clean. 12+3 = 15 Dad says "I cleaned that room" and writes "undo" on the chart. Mum calculates: 15 - (+3) = 12 Dad sees Ally brushing the dog. Writes "+3" on the chart. Mum calculates: 12 + (+3) = 15 Ally throws a stone against the window. Dad writes "-3" on the chart. Mum calculates: 15 + (-3) = 12

See: both "15 - (+3)" and "15 + (-3)" result in 12.

So:

It doesn't matter if you subtract positive points
or add negative points,
you still end up losing points.

Now, Here Are The Rules

	Rule		Example
	Two like signs become a positive sign	+(+)	3+(+2) = 3 + 2 = 5
		-(-)	6-(-3) = 6 + 3 = 9
	Two unlike signs become a negative sign	+(-)	7+(-2) = 7 - 2 = 5
		-(+)	8-(+2) = 8 - 2 = 6

Try These Exercises ...

Now try This Worksheet, and see how you go.

... or Play With It!

Try playing Casey Runner, you need to know the rules of positive and negative to succeed!